1. Statement of prior art
A known receiver for achieving this uses two receiving channels. Each receiving channel includes a mixer and received signals are fed to the mixers in phase quadrature to one another. A local oscillator feeds a mixing signal to each mixer, the mixing signals being at a frequency which is midway between the frequencies transmitted for the logical "one" and "nought". Output signals from the two mixers are filtered, amplified and fed to a detector which provides output signals representative of the transmitted data. Such a two channel system has been described in U.K. Patent Specification No. 1172977.
The use of two separate receiving channels results in duplication of components and in a receiver which is complex and costly.
In United Kingdom Patent Application No. 2076238 there is described a radio receiver for receiving frequency shift keyed radio signals in which only a single receiving channel is used.
The invention of the above-mentioned application provides a radio receiver for receiving frequency shift keyed (FSK) radio signals in which differing binary bits are transmitted at respective ones of two frequencies; the receiver comprising a mixer for receiving the transmitted signals; a local oscillator arranged to feed a mixing signal to the mixer, a detector arranged to be fed with an I.F. signal from the mixer through a single receiving channel, a chopper arranged to apply a chopping signal to the local oscillator to change the frequency of the oscillator at a predetermined rate between the two values none of which lies midway between the two frequencies, the chopping signal being fed, in operation, to the detector in synchronism with that applied to the local oscillator whereby the detector is operative to detect the transmitted binary bits.
At any instant the mixer will provide either a relatively high frequency tone or a relatively low frequency tone in dependence upon the transmitted data at that instant and the frequency of the local oscillator. If the chopper output is such as to set the local oscillator to the higher of its two frequencies and the mixer provides an output which is also the relatively high frequency tone then the frequencies of the local oscillator and the received signal are at their furthest apart. If the mixer output is at the relatively low frequency whilst the local oscillator is at its high frequency then the transmitted frequency of the received signal and the local oscillator frequency are at their closest together.
The detector is operative to compare the chopper signal with the mixer output in logical fashion and provides an output signal which represents the transmitted data.
The above described single channel receiver provides an advantage in switching the frequency of the local oscillator in that a single channel is effectively time division multiplexed so that it operates as two channels. This provides a significant reduction in cost and complexity.
A problem arises with the above described single channel receiver when operation is required at higher data rates. It is essential for the receiver to operate satisfactorily that the mixer provides an output which is one or other of two distinct tones. As the data rate is increased a point is reached when the mixer output ceases to be a discrete tone and becomes a band of frequencies.
2. Summary Of The Invention
This invention seeks to provide a receiver for receiving FSK signals which utilises a single receiving channel and in which the above-mentioned disadvantage is mitigated.
According to one aspect of this invention there is provided a radio receiver for receiving frequency shift keyed (FSK) radio signals in which differing binary bits are transmitted at a different one of two transmission frequencies, the receiver comprising a mixer having a signal input for receiving the transmission frequencies, an input for a local oscillator and an output, a local oscillator having an output coupled with the local oscillator input of the mixer, which local oscillator has a switching signal input and is switchable between two frequencies neither of which is midway between the two transmission frequencies, a detector having an input coupled with the output of the mixer and a switching output coupled with the switching input of the local oscillator which detector is arranged to determine when the output from the mixer contains a frequency which is the difference between one of the two transmission frequencies and one of the local oscillator frequencies or the other of the transmission frequencies and the other of the local oscillator frequencies and to provide a switching signal for switching the local oscillator to its alternative frequency when such a frequency is detected, whereby the switching signal follows the transmitted binary bits and is coupled to a receiver output to provide a signal representative of the transmitted binary information.
According to another aspect of the invention there is provided a radio receiver for receiving frequency shift keyed (FSK) radio signals in which differing binary bits are transmitted at a different one of two transmission frequencies, the receiver comprising a mixer having a signal input for receiving the transmission frequencies, an input for a local oscillator and an output, a local oscillator having an output coupled with the local oscillator input of the mixer, which local oscillator has a switching signal input and is switchable between two frequencies neither of which is midway between the two transmission frequencies, a detector having an input coupled with the output of the mixer and a switching output coupled with the switching input of the local oscillator which detector is arranged to determine when the output from the mixer contains a frequency which is the difference between one of the two transmission frequencies and one of the local oscillator frequencies or the other of the transmission frequencies and the other of the local oscillator frequencies and to provide a switching signal for switching the local oscillator to its alternative frequency in the absence of detection of such a frequency, whereby in the presence of a signal, the switching signal follows the transmitted binary bits and is coupled to a receiver output to provide a signal representative of the transmitted binary information.
The receiver may include muting means responsive in the absence of a received signal to inhibit switching of the local oscillator frequency.
Alternatively, the switching signal may be coupled to the receiver output via switch means and the receiver may include a muting means responsive in the presence of a received signal to close the switch and to open the switch in the absence of a received signal.
In one advantageous form of the invention the oscillator frequencies are equidistantly displaced one each side of the arithmetic mean of the two transmission frequencies whereby said differences between one of the two transmission frequencies and one of the local oscillator frequencies and between the other of the transmission frequencies and the other of the local oscillator frequencies are equal.
In order to reduce the risk of adjacent channel interference the local oscillator frequencies may each be in a frequency band the extremities of which are defined by the two transmission frequencies.
The receiver may include a filter coupled between the output of the mixer and the input of the detector and having a pass band arranged to permit passage of the frequencies to be detected and to reject higher frequencies. The filter may be a low pass filter.
The detector may include a frequency detector circuit for sensing the frequency of the signals derived from the mixer and in one embodiment of the invention the frequency detector circuit is a digital frequency detector.
A flip-flop may be provided coupled to the frequency detector and operative to change state each time the frequency detector senses that the frequency of the signals derived from the mixer has changed from the predetermined value.
The flip-flop may be coupled to the local oscillator to effect switching of the local oscillator at each change of state of the flip-flop.